A carbonylation process of a methanol method is known as an industrial method for producing acetic acid. In this process, for example, methanol and carbon monoxide are reacted in the presence of a catalyst in a reaction vessel to produce acetic acid. The obtained reaction mixture is separated in an evaporator into a vapor phase containing acetic acid and lower boiling point components and a residual liquid phase containing acetic acid and the catalyst. The residual liquid phase is recycled to the reaction vessel while the vapor phase is separated by distillation into lower boiling point components and crude acetic acid. The crude acetic acid is further purified to obtain product acetic acid.
This carbonylation process of the methanol method has the major advantage that acetic acid can be efficiently produced from inexpensive raw materials, but presents the problem that the catalyst is easily precipitated. In this process, the prevention of the catalyst precipitation is an important challenge from the viewpoint of securing productivity by the maintenance of a catalyst concentration in the reaction vessel, and securing safety operation by the prevention of catalyst concentration variation in the reaction vessel. Among others, the most important challenges are to prevent catalyst precipitation in an evaporator where the catalyst is most easily precipitated, and to improve the rate of catalyst recycle from the bottom of the evaporator to the reaction vessel.
It has heretofore been known that a catalyst stabilizer such as lithium iodide is contained in a reaction solution in order to prevent catalyst precipitation. Also, a technique of improving the stability of carbon monoxide and a complex of an iodide ion and a metal is known which involves introducing a carbon monoxide-containing gas to the bottom of an evaporator to increase a CO partial pressure or the amount of CO dissolved in an evaporator bottom fraction. For example, Patent Literature 1 discloses a method for preventing catalyst precipitation in an evaporation step by introducing a carbon monoxide-containing gas stream to a separation zone in the step of transferring a carbonylation product solution to the separation zone maintained at a pressure lower than that of a reaction zone, subjecting a portion of the carbonylation product to flash separation, and circulating an unflashed carbonylation product solution to the reaction zone.